The present invention relates to an optical recording medium of a write once type and a method for writing and reading information on and from the recording medium, and more particularly to the medium which involves power calibration of a laser beam upon writing.
As a write once type optical recording medium, a CD-R (write once type CD) is known. In the CD-R, a laser beam is focused on a recording surface to convert luminous energy to thermal energy to change the physical nature of the recording surface. Thus, the information is recorded on the CD-R.
Although the recording mediums of the same type are made by the same material, the individual recording medium has not always the same characteristic. Accordingly, if a laser beam is set to a fixed power, information can not be recorded in an optimum condition. Therefore, in such a recording medium, before recording information, the laser power is adjusted by an optimum power control (OPC) to be set to an optimum value for a disc to be recorded.
A conventional OPC method employed in the CD-R will be described with reference to FIG. 7. The CD-R has a power calibration area (PCA) as a test area, lead-in area LI, information recording area DATA, and lead-out area LO. The PCA is provided on the inside of the lead-in area. A test writing is performed in the test area before recording information so as to obtain an optimum laser power for writing information on the CD-R.
More particularly, in order to determine the optimum laser power, in the PCA is recorded a test data, the power of which changes step by step within a predetermined range. When the test data is reproduced, the optimum recording power can be selected depending on the condition of the reproduced data. For example, as shown in FIG. 6a, an EFM pulse signal having a pulse width such as 3T and 11T is used to stepwisely change the power of the laser beam emitted from a pickup, thereby recording the test data. When the test data is reproduced, the frequency level and the reproduced waveform are checked to determine the optimum power.
The test data need not be confined to 3T and 11T data but may be other modulated data. The modulated data are recorded in the same format as the ordinary data.
In the write once type CD such as the CD-R, an area in which information has been written once is not used for further writing information again. A recorder for writing information is provided for controlling the laser power by the test writing in the PCA, and for recording information in the information recording area DATA thereafter with the controlled laser power.
As shown in FIG. 7, the PCA is divided into about a hundred partitions or blocks. Since one writing test uses one block, it is possible to perform a hundred times of the OPC.
However, the test writing over a hundred times can not be done. Therefore, even if a writable area (vacant space) remains in the information recording area, information can not be recorded therein.
Another drawback of the conventional recording method is caused by the fact that the characteristic of the recording layer is not even throughout the entire medium. In the conventional system, the PCA, wherein the test data is recorded, is inside the lead-in area LI so that the area is often distant from the information recording area DATA. The two areas may have rather different characteristics. Hence the determined laser power appropriate for the PCA may not always be appropriate for the recording area. In order to restrain the drawback, it is necessary to elaborately control the production of the recording layer at manufacturing thereof to render the characteristic of the layer even. Thus the manufacturing cost of the optical disc cannot be reduced.
In addition, it would be advantageous if the test data for power calibration, a procedure of which cannot be omitted at recording, is used to prevent illicit copying of data.